1. Field of the Invention
The invention relates to a power supply apparatus and an image forming apparatus, and particularly to a power supply apparatus suitable as the power source of an image forming apparatus such as a copying machine or a printer using an electrophotographic process, and an image forming apparatus using the power supply apparatus.
2. Description of Related Art
Due to the rapid advance of digital technology and semiconductor integrated circuit technology in recent years, it has been practiced to digitize the control of the switching power supply of a copying machine and printer and a high voltage source, make a chip for control and mount it in a power supply apparatus.
An example of the conventional art will hereinafter be schematically described with reference to FIGS. 1, 2, and 3 of the accompanying drawings.
Description will be made here with attention paid only to a high voltage output called a high voltage A. When there are a plurality of outputs, time division control is effected by the use of a multiplexer or the like, whereby a plurality of outputs are also realized.
Each part will first be described. An inverter transformer T1 converts a voltage inputted to a primary side in conformity with the turn ratio between the primary side and a secondary side and outputs a predetermined voltage to the secondary side. A switching element Q4 drives the inverter transformer T1. A sample hold circuit 9 latches an input signal for a predetermined period. An A/D converter 10 converts an analog signal into a digital signal. A comparator 11 outputs the result of the comparison of the magnitude of two inputs. A pulse width modulation (PWM) circuit 13 prepares a pulse signal (PWM signal) of an ON/OFF time ratio conforming to the value of an up-down counter 12. A controller 3 governs the control of each element.
Description will now be made of the connection of each part and element and a series of operations.
(a) High Voltage Producing Means 1
A predetermined voltage as B-voltage is inputted to the primary side input of the inverter transformer T1, and the other end of the inverter transformer T1 is connected to the switching element Q4. A pulse signal (PWM signal) is inputted from the PWM circuit 13 in high voltage control means 2 which will be described later to the gate of the switching element Q4. The switching element Q4 performs a switching operation in conformity with the PWM signal to thereby obtain a pulse output conforming to the turn ratio and the PWM signal on the secondary side of the inverter transformer T1. The pulse output is rectified by a diode D301 and a capacitor C201, and a DC high voltage output is produced on the cathode side of the diode D301. A boosting and rectifying circuit 1-2 and driving means 1-1 are generally called fly-back type (or ON/OFF type) converters, and output a higher voltage as the ON time percentage in the switching operation of the switching element Q4 is greater.
Resistors R101 and R102 are connected to the high voltage output, and also a voltage dropped by the resistance division of each resistor, i.e. the detection signal of the high voltage output, is outputted.
(b) High Voltage Control Means 2
The high voltage output detection signal obtained by the resistors R101 and R102 in the high voltage producing means 1 is inputted to the sample hold circuit 9. The sample hold circuit 9 holds the high voltage output detection signal for a predetermined period. The high voltage output detection signal of the A/D converter 10 is converted into digital data and is inputted to one of the input terminals of the digital comparator 11.
A sequence control unit 200 for controlling a high voltage unit 100 outputs digital data corresponding to the output set value (target value) of a high voltage output A. This target value is inputted to the other input terminal of the digital comparator 11.
The digital comparator 11 compares the high voltage output detection signal converted into the digital data with the target value given from the sequence control unit 200. When the high voltage output detection signal is smaller than the target value, the output of the up-down counter 12 is made greater by a predetermined number, and when the high voltage output detection signal is greater than the target value, the output of the up-down counter 12 is made smaller by the predetermined number. The PWM circuit 13 produces an appropriate PWM signal in conformity with the value of the up-down counter 12. The PWM signal is a driving signal for the switching element Q4 in the high voltage producing means 1, and is a signal for determining the switching time percentage of the switching element Q4. When the output of the up-down counter 12 becomes great, the time percentage for which the switching element Q4 is switched on is made great to thereby make the high voltage produced by the high voltage output producing means 1 great. Also, when the count value which is the output of the up-down counter 12 becomes small, the time percentage for which the switching element Q4 is switched on is made small to thereby make the high voltage produced by the high voltage producing means small. By the above-described operation, the high voltage output A is controlled to a predetermined value conforming to the target value. On the other hand, sequence control and image processing in an image forming apparatus such as a copying machine or a printer is complicated, and the scale of an application specific integrated circuit (ASIC) exclusively therefor is increased. The capacity of the exclusive ASIC has become much larger as compared with the capacity of a control chip used for the control of a power source.
So, even if as shown as an example of related art in FIG. 4 of the accompanying drawings, a power source control chip portion (high voltage control means 2) is added to the “extra portion” of the aforedescribed large-scale ASIC exclusively for sequence control/image processing, the scale of the entire circuit is not changed greatly, nor the cost is changed.
The mounted position of the ASIC exclusively for sequence control/image processing, however, is on the sequence control unit 400 or an image processing unit, and these units are units discrete from a switching power source and a high voltage source unit 300, and the mounted position in the image forming apparatus often becomes separate. So, the sequence control unit 400 or the image processing unit and the switching power source and the high voltage source unit 300 are connected together by bundle lines or the like, and the number of signal lines becomes great. Mentioning the high voltage source of the latest color copying machine as an example, there are necessary high voltage outputs for four colors in each of charging (AC+DC), developing (AC+DC) and transferring, and sheet separation and further, auxiliary charging, auxiliary transferring, cleaning, charge removing, etc. as required, and the number of bundle lines reaches the number of bundle lines corresponding to the drive signal, the detection signal, etc. of the respective outputs. Consequently, it is conceivable to digitize the drive information and detection information of each output and effect time division control by serial communication, but in such case, it becomes necessary to mount an encoder/decoder and digital circuits such as a serial communication circuit and a handshake also in the interior of the high voltage source. Thus, even if the power source control chip portion is added to the “extra portion” of the ASIC exclusively for sequence control/image processing, the aforementioned digital circuits further exist in the high voltage source portion, and any cost merit cannot be provided.